A photovoltaic (PV) cell converts energy in light incident upon a light-absorbing layer in the PV cell to electrical current and voltage. A single PV cell with a light-absorbing layer fabricated from silicon has an open-circuit output voltage of about 0.5 to about 0.7 volt cell and an output current related to an amount of surface area available for absorbing incident light, cell temperature, and other factors. Two or more PV cells may be connected together electrically to form a PV module having higher output voltage and more output current than a single PV cell. For example, PV cells may be connected to one another with series and parallel electrical connections to form a PV module having an electrical output power of about 40 watts in a mechanical support structure approximately 25 inches long by 20 inches wide by 2 inches thick, with many other sizes and power ratings available for PV modules. A PV module may include different layers for protecting PV cells in the module from dirt, exposure to water, and mechanical stress, and may include electrical terminals for connecting the PV module to other PV modules or to an electrical load.
Silicon used in the production of PV cells may be subjected to high processing temperatures for refining and annealing raw materials and wafers. Alternative PV cell technologies are being explored which take advantage of lower processing temperatures, possibly saving energy during cell manufacture and permitting the use of low-cost manufacturing processes and materials that may be unable to withstand high processing temperatures. For example, PV cells having a light-absorbing layer including many small, colloidal semiconductor quantum dots (QDs) may reduce manufacturing costs significantly compared to PV cells made from silicon wafers sliced from a silicon boule or ribbon. QDs may be formed by wet chemistry methods in which approximately spherical nanoparticles of a light-absorbing compound such as lead sulfide (PbS) or another semiconductor compound are synthesized in a liquid solution and deposited as a granular thin film on a solid surface. QD synthesis and deposition may be performed at or near room temperature, much lower than temperatures for manufacturing silicon wafers.
The band gap energy of a quantum dot (QD) is related to the size of the QD. The size of a QD may be represented by a linear dimension of the QD, for example a diameter of the QD. An individual QD in a QD film for a PV cell may have a diameter in a range from a few nanometers to a few tens of nanometers. The power conversion efficiency of a PV cell may be maximized at a selected wavelength of incident light by controlling the size of the QDs forming the light absorbing layer in the PV cell.
Long-chain ligands extending from the surface of a QD may act as electrical insulators that reduce the mobility of charge carriers between QDs in a light-absorbing layer in a PV cell. Long-chain ligands bonded to QDs may therefore reduce the electrical power conversion efficiency of a PV cell, where power conversion efficiency ηp may be defined as the product of open-circuit voltage Voc, short-circuit current Jsc, and fill factor FF as shown in equation (1).ηp=Voc×Jsc×FF  (1)Fill factor may be defined as the ratio of the maximum power from the PV cell to the product of Voc and Jsc as shown in equation (2).FF=(Imp×Vmp)/(Voc×Jsc)  (2)Imp refers to the current output from the PV cell at the cell's maximum output power and Vmp refers to the output voltage at maximum output power.
Exchanging long-change ligands for shorter ligands may improve the power conversion efficiency of a PV cell having QDs in a light-absorbing layer. Ligand exchange may be performed repeatedly during synthesis or during deposition of QDs to replace long-chain ligands with shorter ligands throughout the volume of material included in each QD. Ligand exchange may reduce the volume of a QD, thereby affecting electrical parameters of a device incorporating QDs, and may cause some kinds of defects that interfere with energy conversion and with electrical current flow between QDs in a QD film. A defect may arise from abrupt termination of atoms on the surface of a QD or from an undesirable atomic ratio.
Performing ligand exchange during the QD deposition process may increase uncertainty in parameters related to performance of a finished PV cell, lengthen manufacturing time, and increase manufacturing cost for PV cells. Defects in a QD may degrade one or more of the parameters Jsc, Voc, and FF and may reduce the power conversion efficiency and an amount of electrical power output from a PV cell. PV cells could be manufactured at lower cost for a specified power conversion efficiency compared to previously known methods if defects in QDs could be repaired after deposition of the QDs on a substrate.